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A volume label is an identifier that you can assign to a volume (partition or drive). The volume label is displayed in Windows's My Computer (the name varies with the version) and in Windows Explorer. You can also retrieve it from the command line using vol. The volume label is intended as an identifier for the user.

Although it is not recommended, you can have several volumes with the same label at once. Volume labels are especially useful for removable media, since the assigned drive letter may vary. For example, if you're using to USB flash drives at once, the volume label provides an easy way of knowing which one is which.

The drive letter (much like mount points in Linux and modern versions of Windows) of a volume is an identifier inside the root filesystem namespace. It is needed by the operating system to access the contents of the volume.

You cannot assign the same drive letter (or mount point) to two volumes at once. However, removable media of the same type may share the same drive letter. In particular, all disks inserted in the same DVD drive will use the same drive letter.

Every operating system needs a way to interact with connected volumes. Each volume has its unique identifier and a possibly name.

Names aren't unique, so it's not a good idea to use them to distinguish drives. On the other hand, UUIDs aren't flexible enough - for example you may want to replace a volume with another one, but keeping it accessible the same way as before. It wouldn't work with UUIDs.

So OSes must have some way of mapping volumes to other kind of identifiers. Windows uses drive letters for this: you can assign any not-yet-used drive letter to any drive, possibly more that one letter to a single drive. That letter isn't reserved permanently, it will be assignable again as soon as you disconnect that volume.

Unix-like systems are based on a mounting scheme (which is also partially implemented in Windows NTFS filesystem). One volume becomes a root (/) and other volumes can be virtually mounted in subdirectories of that volume (usually subdirectories of /mnt or /media are used). This scheme is much more powerful than drive letters, as you are not limited by alphabet letters and can give your volumes any virtual names.

"A volume is not the same thing as a partition. For example, a floppy disk might be accessible as a volume, even though it does not contain a partition, as floppy disks cannot be partitioned with most modern computer hardware. Also, an OS can recognize a partition without recognizing any volume associated with it, as when the OS cannot interpret the filesystem stored there. This situation occurs, for example, when Windows NT-based OSes encounter disks with non-Microsoft OS partitions, such as the ext3 filesystem commonly used with Linux. "

volume label is more general alternate identifier to UUIDs. You can think of drive letters as mount points where a volume under a label can be assigned to differen drive letters and they specific to Windows